Method for making Ni-Si magnetron sputtering targets and targets made thereby

ABSTRACT

A method for making a nickel/silicon sputter target, targets made thereby and sputtering processes using such targets. The method includes the step of blending molten nickel with sufficient molten silicon so that the blend may be cast to form an alloy containing no less than 4.5 wt % silicon. Preferably, the cast ingot is then shaped by rolling it to form a plate having a desired thickness. Sputter targets so formed are capable of use in a conventional magnetron sputter process; that is, one can be positioned near a cathode in the presence of an electric potential difference and a magnetic field so as to induce sputtering of nickel ion from the sputter target onto the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 09/509,360, filed Mar. 24, 2000, now U.S. Pat. No. 6,423,196,which, in turn was a U.S. national phase filing under 35 USC §371 ofInternational PCT Application No. PCT/US98/24983, filed Nov. 19, 1998,which claimed priority filing benefit from U.S. Provisional ApplicationSerial No. 60/066,185 filed Nov. 19, 1997.

FIELD OF THE INVENTION

The present invention relates to methods for making sputter targets formagnetron sputtering, sputter targets made by the methods, and methodsof sputtering using such targets. More particularly, the inventionrelates to the manufacture of sputter targets using nickel-siliconalloys and to targets manufactured thereby.

BACKGROUND OF THE INVENTION

Cathodic sputtering is widely used for depositing thin layers or filmsof materials from sputter targets onto desired substrates such assemiconductor wafers. Basically, a cathode assembly including a sputtertarget is placed together with an anode in a chamber filled with aninert gas, preferably argon. The desired substrate is positioned in thechamber near the anode with a receiving surface oriented normally to apath between the cathode assembly and the anode. A high voltage electricfield is applied across the cathode assembly and the anode.

Electrons ejected from the cathode assembly ionize the inert gas. Theelectrical field then propels positively charged ions of the inert gasagainst a sputtering surface of the sputter target. Material dislodgedfrom the sputter target by the ion bombardment traverses the chamber anddeposits on the receiving surface of the substrate to form the thinlayer or film.

In so-called magnetron sputtering, one or more magnets are positionedbehind the cathode assembly to generate a magnetic field. Magneticfields generally can be represented as a series of flux lines, with thedensity of such flux lines passing through a given area, referred to asthe “magnetic flux density,” corresponding to the strength of the field.In a magnetron sputtering apparatus, the magnets form arch-shaped fluxlines which penetrate the target and serve to trap electrons in annularregions adjacent the sputtering surface. The increased concentrations ofelectrons in the annular regions adjacent the sputtering surface promotethe ionization of the inert gas in those regions and increase thefrequency with which the gas ions strike the sputtering surface beneaththose regions.

Nickel is commonly used in physical vapor deposition (“PVD”) processesfor forming nickel silicide films by means of the reaction of depositednickel with a silicon substrate. Yet, while magnetron sputtering methodshave improved the efficiency of sputtering many target materials, suchmethods are less effective in sputtering “ferromagnetic” metals such asnickel. It has proven difficult to generate a sufficiently strongmagnetic field to penetrate a nickel sputter target to efficiently trapelectrons in the annular regions adjacent the sputtering surface of thetarget.

In order to provide a background for the present invention, certainaspects of the magnetic behavior of metals will be briefly described.

The magnetic flux density vector within a metal body generally differsfrom the magnetic flux density external to the body. Typically, thecomponent “B” of the magnetic flux density along a given direction inspace within a metal body may be expressed in accordance with therelationship B=μ₀(H+M), where “μ₀” is a constant referred to as themagnetic permeability of empty space; “H” is the corresponding componentof the so-called “magnetic field intensity” vector; and “M” is thecorresponding component of the so-called “magnetization” vector. (Notethat positive and negative values of the components of the magnetic fluxdensity, the magnetic field intensity and the magnetization representopposite directions in space, respectively.)

The magnetic field intensity may be thought of as the contribution tothe internal magnetic flux density due to the penetration of theexternal magnetic field into the metallic body. The magnetization may bethought of as the contribution to the internal magnetic flux density dueto the alignment of magnetic fields generated primarily by the electronswithin the metal.

In “paramagnetic” materials, the magnetic fields generated within themetal tend to align so as to increase the magnetic flux density withinthe metal. Furthermore, the magnetic fields generated within aparamagnetic metal do not strongly interact and cannot stabilize thealignment of the magnetic fields generated within the metal, so that theparamagnetic metal is incapable of sustaining any residual magneticfield once the external magnetic field is removed. Thus, for manyparamagnetic metals and at a constant temperature, the “magnetizationcurve,” which relates the magnetic flux density to the magnetic fieldstrength within the metal, is linear and independent of the manner inwhich the external magnetic field is applied.

In a “ferromagnetic” metal such as nickel, the magnetic fields generatedwithin the metal do interact sufficiently for the metal to retain aresidual magnetic field when the external field is removed. Below a“Curie temperature” characteristic of a ferromagnetic metal, the metalmust be placed in an external magnetic field directed oppositely to theresidual field in the metal in order to dissipate the residual field.

At any constant temperature below the Curie temperature, therelationship between the magnetic flux density and the magnetic fieldintensity in the metal differs depending on how the external magneticfield has varied over time. For example, if a ferromagnetic metal ismagnetized to its maximum, or “saturation,” flux density in onedirection in space and then the external magnetic field is slowlyreversed to the opposite direction, the magnetic flux density within themetal will decrease as a function of the magnetic field intensity alonga first path until the magnetic flux within the metal reaches thenegative of the saturation value. If the external field is againreversed so as to remagnetize the metal in the original direction, themagnetic flux density within the metal will increase as a function ofthe magnetic field intensity along a second path which differs from thefirst path in relation to the reversal of the residual magnetic field.The shape of the resulting dual-path magnetization curve, which isreferred to as a “hysteresis loop,” is characteristic of ferromagneticbehavior.

When a ferromagnetic metal is surrounded by a gas in the presence of amagnetic field, the ferromagnetic metal tends to “attract” the fluxlines of the magnetic field away from the surrounding gas into itself.This prevents the flux lines from penetrating the ferromagnetic metaland extending through to the surrounding gas. While paramagnetic metalsmay “attract” some flux lines of an external magnetic field, they do soto a far lesser degree than do ferromagnetic materials.

Above their Curie temperatures, nominally ferromagnetic metals behave ina manner similar to paramagnetic materials. In particular, nominallyferromagnetic metals tend to “attract” far less of the flux of anexternal magnetic field into themselves above their Curie temperaturesthan below.

Thus, without wishing to be bound by any theory of operation, it isbelieved that a nickel sputter target placed in the magnetic field of amagnetron sputtering device tends to “attract” the flux of the magneticfield into itself. This prevents the magnetic flux from penetratingthrough the target, thereby reducing the efficiency of the magnetronsputtering process.

Typically, only thin nickel targets of about 0.12 inch (3 mm) or lesscould be used in magnetron sputtering processes due to the ferromagneticcharacter of nickel. This increases the difficulty and cost ofsputtering nickel, since it is necessary to replace the sputter targetsat frequent intervals.

Meckel U.S. Pat. No. 4,229,678 sought to overcome this problem byheating the target material to its Curie temperature and magnetronsputtering the material while in such a state of reduced magnetization.Meckel further proposed a magnetic target plate structured to facilitateheating of the plate to its Curie temperature by the thermal energyinherent in the sputtering process. One drawback to this proposed methodwas the increased cost inherent in providing for the heating of thetarget as well as providing for the stability of the cathode assembly atincreased temperatures.

The problem of magnetron sputtering nickel has been addressed in thespecialty media industry by alloying the nickel with another transitionmetal such as vanadium. At about 12 at. % vanadium, the alloy ceases tobehave ferromagnetically. Alloys of nickel with other transition metalssuch as chromium, molybdenum and titanium have shown a loss offerromagnetic behavior at compositions under 15 at. %. Adopting asimilar approach, Wilson U.S. Pat. No. 4,094,761 proposed alloyingnickel with copper, platinum or aluminum to produce an alloy having aCurie temperature below the sputtering temperature. Unfortunately, allof these methods share the drawback that the metals alloyed with thenickel constitute impurities when the sputter target is used in a nickelsilicidation process.

Therefore, there remains need in the art for a method for making anickel sputter target which is compatible with magnetron sputteringprocesses.

SUMMARY OF THE INVENTION

These and other objects of the invention are met by a method for makinga nickel/silicon sputter target including the step of blending moltennickel with sufficient molten silicon so that the blend may be cast toform an alloy containing no less than 4.5 wt % silicon, preferably about4.5-50 wt % Si. The cast ingot is then shaped by rolling it to form aplate having a desired thickness and then the rolled plate is machinedto form the desired target shape. The sputter target so formed iscapable of use in a conventional magnetron sputter process; that is, itcan be positioned near a cathode in cathodic sputtering operations, inthe presence of an electric potential difference and a magnetic field soas to induce sputtering of nickel ion from the sputter target onto thesubstrate. However, these targets can be made thicker than conventionalNi targets so that they may be used for longer sputtering times withoutreplacement.

Nickel-silicon alloy sputter targets in accordance with the inventionhave been found to exhibit sufficiently low Curie temperatures thattheir behavior at conventional sputtering temperatures is thoughtparamagnetic rather than ferromagnetic. Thus, the magnetizations oftargets having thicknesses as large as 0.5 inch (1.3 cm) aresufficiently low that the targets may be used in conventional magnetronsputtering processes. Furthermore, the nickel/silicon alloy does notintroduce any impurities when the target is used for nickelsilicidation.

In addition, it has been found that rolling the ingot formed fromcasting the nickel-silicon alloy before machining the target promotesthe deposition of a uniform layer of nickel silicide during thesputtering process.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing the magnetization curves for cast nickel,nickel-2.9 wt % silicon and nickel-4.5 wt % silicon ingots;

FIG. 2 is a chart showing the penetration of magnetic flux throughnickel and nickel-4.39 wt % silicon plates of various thicknesses; and

FIG. 3 is a schematic diagram illustrating the face-centered cubicstructure of pure nickel metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with an especially preferred method for making a sputtertarget, nickel and silicon are blended as powders or small blocks in acrucible and melted in an induction or resistance furnace. Preferably,the blend is then cast to form an ingot containing at least about 4.5 wt% silicon. The ingot is rolled to form a plate having a desiredthickness (i.e., greater than 0.12 inch (3 mm)). Finally, the plate ismachined to form the target.

The nickel and silicon may be blended either in the form of powders orof small blocks. Preferably, the blending occurs in a crucible, whichmay be inserted into an induction or resistance furnace to melt thenickel and silicon. For example, the nickel may be introduced in theform of 1 cubic inch blocks which are melted in a crucible beforeblending with the silicon.

The casting, rolling and machining of the metal may be carried out byconventional means well known to those of ordinary skill in the art.

The alloy should contain sufficient nickel to form an effective nickelsilicide film when sputtered. Thus, it is preferred that the alloy notexceed an upper limit, perhaps on the order of 50 wt %.

The invention will be further described by means of the followingexamples, which are illustrative only and not limitative of theinvention as claimed.

EXAMPLE 1

Three 10 g blends of nickel and silicon powders were prepared, melted incrucibles and cast to form pure nickel, nickel-2.9 wt % silicon andnickel-4.5 wt % silicon alloy ingots. Differential thermal analyses wereperformed to verify these compositions. After the compositions wereverified, a VSM was used to obtain the magnetization curves for each ofthe three compositions.

The results are shown in FIG. 1. In the chart shown in FIG. 1, thehorizontal axis 10 represents the magnetic field intensity (“H”) withinthe ingot while the vertical axis 12 represents the magnetic fluxdensity (“B”) within the ingot. The magnetization curve 20 correspondsto the pure nickel ingot and the magnetization curve 22 corresponds tothe nickel-2.9 wt % silicon alloy. While the saturation magnetic fluxdensity of the nickel-2.9 wt % silicon alloy is about half thesaturation level of the pure nickel, both of the magnetization curves20, 22 feature significant hystersis indicative of ferromagneticbehavior.

On the other hand, the magnetization curve 24, which corresponds to thenickel-4.5 wt % silicon alloy, exhibits no significant hysteresis andappears approximately linear with a gentle slope. Thus, themagnetization curve 24 shows that the behavior of the nickel-4.5 wt %silicon alloy was paramagnetic rather than ferromagnetic. This result,along with the gentle slope of the magnetization curve 24, implies thatthe nickel-4.5 wt % silicon alloy is a suitable target material for amagnetron sputtering process.

EXAMPLE 2

In order to illustrate this further, a nickel-silicon alloy ingot wasformed by melting and casting a blend of nickel and silicon powders. Thecomposition of the alloy was shown by atomic absorption to benickel-4.39 wt % silicon. Under variable source magnetometry, this alloywas found to exhibit slight hysteresis, as shown in FIG. 1 by itsmagnetization curve 26.

Plates of various thicknesses were prepared from pure cast nickel andfrom the nickel-4.39 wt % silicon alloy. The percentage of the magneticflux which penetrated through these plates was then measured as afunction of the plate thickness.

The results are shown in FIG. 2. The horizontal axis 30 in FIG. 2represents plate thickness in inches while the vertical axis 32represents the measured percentage of the original flux density whichpenetrated the plate. The line 40 represents the penetration of themagnetic flux through the nickel plates while the line 42 represents thepenetration through the nickel-4.39 wt % silicon plates. Atapproximately 0.14 inch (3.5 mm) thickness, the magnetic penetration ofthe nickel was approximately 71%. By way of comparison, the magneticpenetration of the nickel-4.39 wt % silicon alloy at a thickness of0.125 inch (3.2 mm) was greater than 96%.

Even at a thickness of 0.5625 inch (1.53 cm), approximately 90% of themagnetic flux penetrated through the nickel-4.39 wt % silicon alloy.While the magnetic flux penetration through the nickel-4.39 wt % siliconalloy decreased approximately linearly with increasing thickness, theseresults imply the suitability of sputter targets, made in accordancewith the invention and having thicknesses as great as 0.5 inch (1.5 cm),for use in magnetron sputtering processes.

EXAMPLE 3

A nickel-4.5 wt % silicon ingot was cast and rolled to a thickness of3.5 inch (8.9 cm). Slices were cut before and after the rolling processfor scanning electron microscopy/optical microstructure analysis(“SEM”). An X-ray diffraction (“XRD”) study also was made of the rolledsample. In addition, sputter targets having a 3 inch (7.6 cm) diameterswere machined from the alloy before and after rolling.

By way of background, a metallic sputter target typically comprises aplurality of “grains” of a size visible under an optical microscope.Within each grain, the metal atoms align in a crystalline matrix.

Pure nickel typically crystallizes in a “face-centered cubic” matrix.Each nickel atom in the face-centered cubic matrix is typicallysurrounded by twelve other equally spaced nickel atoms. As shown in FIG.3, the crystalline structure of pure nickel can be illustrated by meansof a so-called “unit cell” 50 which includes a first set of nickel atoms52 at each of the corners of an imaginary cube 54 and a second set ofnickel atoms 56 centered on the faces of the imaginary cube 54.

An indication of the true size of the unit cell 50 is given by the“lattice parameter,” which is the length of one of the sides of theimaginary cube 54. The lattice parameter for a unit cell of pure nickelis approximately 3.524 Å.

Two sets of planes relative to the crystalline matrix are specificallyindicated on the unit cell 50 of FIG. 1: so-called “(200) planes”parallel to the sides of the imaginary cube 54 and so-called “(111)planes” which form diagonals relative to the sides of the cube 54.Examples of (200) planes are shown at 60 and examples of (111) planesare shown in phantom at 62. Since the unit cell 50 is symmetric withrespect to a center point (not shown) of the imaginary cube 54, each ofthe (200) planes 60 is physically equivalent to each of the other (200)planes. Likewise, each of the (111) planes 62 is physically equivalentto each of the other (111) planes.

Note that the atoms 52, 56 in the unit cell 54 are more closely packedalong the (111) planes 62 than along the (200) planes. The distancebetween these close-packed (111) planes 62 coincides with the so-called“d-spacing” of the lattice, which in the case of pure nickel isapproximately 2.034 Å.

The SEM studies of the slices taken from the nickel-4.5 wt % siliconalloy before and after rolling showed that the grain sizes in theas-rolled slice were more uniform than those in the slice taken prior torolling. It has been found that more uniform grain sizes tend to promotethe deposition of more uniform film during sputtering processes.

The XRD study of the as-rolled material revealed that the materialshowed a preferred (200) orientation as opposed to a (111) orientation.The d-spacing of the as-rolled alloy was found to be 2.0354 Å, whichcorresponds closely to the d-spacing of pure nickel. This latterobservation suggests a minimum of matrix deformation which mightinterfere with the deposition of a uniform film during sputtering.

EXAMPLE 4

The nickel-4.5 wt % silicon sputter targets prepared in Example 3 weremounted on magnetron-type cathodes for use in a DC sputtering system. Abase pressure of 5.7×10⁻⁷ Torr was achieved using a cryopump. Siliconsubstrates were placed below the targets and sputtered at ambient heat.The targets were sputtered in a 6 m Torr argon atmosphere at 150 W DCsputtering power. The flow of argon and the total gas pressure weremanually adjusted by mass flow controllers and monitored with acapacitive manometer. After the sputtering was completed, portions ofthe as-deposited films were annealed under a positive field of argon gasat 400° C. for 30 min. The process was found to produce satisfactorynickel silicide films on the silicon substrates.

The foregoing examples demonstrate that the magnetizations ofnickel-silicon alloy sputter targets in accordance with the inventionhaving thicknesses as large as 0.5 inch (1.3 cm) are sufficiently lowthat the targets may be used in conventional magnetron sputteringprocesses. In addition, it has been found that rolling the ingots formedfrom casting the nickel-silicon alloys before machining the targetpromotes the formation of uniform grain sizes in the alloys, which, inturn, promotes the deposition of uniform layers of nickel silicideduring the sputtering processes. Since no transition metals are alloyedwith the nickel to lower its Curie temperature, no impurities areintroduced when such targets are used in nickel silicidation processes.

While the method described herein and the sputter targets produced inaccordance with the method constitute preferred embodiments of theinvention, it is to be understood that the invention is not limited tothese precise methods and sputter targets, and that changes may be madein either without departing from the scope of the invention, which isdefined in the appended claims.

What is claimed is:
 1. Sputter target exhibiting high magnetic permeability for use in magnetron sputtering methods, said target exhibiting a magnetic penetration of about at least 90%, said target consisting essentially of a Ni/Si alloy and having a thickness of at least 0.125″.
 2. Sputter target as recited in claim 1 wherein said magnetic penetration is greater than about 96%.
 3. Sputter target as recited in claim 1 having a thickness of about 0.5 inches.
 4. Sputter target as recited in claim 1 wherein said target has a thickness of 0.5625 inches.
 5. Sputter target as recited in claim 3 wherein Si is present in an amount of about 4.39 wt % and greater.
 6. Sputter target as recited in claim 5 wherein Si is present in an amount of less than 50 wt %.
 7. Sputter target as recited in claim 1 having d-spacing of about 2.0354 A.
 8. Sputter target as recited in claim 1 having a preferred (200) orientation.
 9. Sputter target comprising a Ni/Si alloy wherein Si is present in an amount of about 4.5 wt % to about 50 wt %, said target having a thickness of about 0.5″ and exhibiting a magnetic penetration of at least 90%.
 10. Method of sputter coating a Ni/Si material onto a silicon wafer comprising: providing a magnetron sputtering system; providing a Ni/Si sputter target, said Si being present in an amount of between about 4.5 wt %-50 wt %; providing a magnetic field through said target, said target permitting at least 90% magnetic penetration of said magnetic field therethrough; and sputtering said Ni/Si material from said target onto said silicon wafer to provide a film consisting essentially of nickel silicide thereon.
 11. Method as recited in claim 10 wherein said target has a thickness of about 0.5″. 